Li-6 and C-13 NMR spectroscopic studies were carded out on [Li-6]n-BuLi and [Li-6]PhLi (RLi) in toluene-d(8) containing the following diamines: N,N,N,N-tetramethylethylenediamine (TMEDA), N,N,N,N-tetraethylethylenediamine, 1,2-dipyrrolidinoethane, 1,2-dipiperidinoethane, N,N,N,N-tetramethylpropanediamine, trans- (R, R)-N,N,N,N-tetramethylcyclohexanediamine, and (-)-sparteine. Dimers of general structure (RLi)(2)S-2 (S = chelating diamine) are formed in each case. Treatment of RLi with two different diamines (S and S affords homosolvates (RLi)(2)S-2 and (RLi)(2)S'(2) along with a heterosolvate (RLi)(2)SS'. Relative binding constants and associated free energies for the sequential solvent substitutions are obtained by competing pairs of diamines. The high relative stabilities of certain heterosolvates indicate that solvent binding to the RLi dimer can be highly correlated. Rate studies of both the 1,2-addition of RLi/TMEDA to the N-isopropylimine of cyclohexane carboxaldehyde and the RLi/TMEDA-mediated alpha-lithiation of the N-isopropylimine of cyclohexanone reveal monomer-based transition structures, [(RLi)(TMEDA)(imine)](double dagger), in all cases. The complex relationships of solvent binding constants and relative reactivities toward 12-additions and alpha-lithiations are discussed.